1. Field of Invention
The invention relates to an exhaust system that purifies exhaust gas using catalyst and converts thermal energy of the exhaust gas into electric energy.
2. Description of Related Art
Generally, an exhaust system includes an exhaust emission catalyst such as a three-way catalyst to purify exhaust gas discharged from an engine by removing hazardous substance contained in the exhaust gas, for example, carbon monoxide, hydrocarbon, nitrogen oxides and the like. The catalyst becomes effective to purify the exhaust gas when it is activated at its activated temperature in the range between 350° C. and 800° C., for example.
Substantially high thermal energy of the exhaust gas at a high temperature is partially used for increasing the temperature of the exhaust emission catalyst until it reaches the activated temperature. The rest of the thermal energy of the exhaust gas, however, is dispersed without being collected. An exhaust heat power generation apparatus has been developed to collect the thermal energy through conversion thereof into electric energy.
In a certain type of the aforementioned exhaust heat power generation apparatus, a thermoelectric converting module is interposed between an exhaust pipe (high temperature side) through which the exhaust gas flows and a cooling unit (low temperature side), and each thermoelectric converting element of the thermoelectric converting module generates power in accordance with the temperature difference between the high temperature side and the low temperature side (Related Art 1: JP-A-10-234194). The temperature difference has to be increased while raising the temperature at the high temperature side so as to improve the thermoelectric conversion efficiency. In the other type of the exhaust heat power generation apparatus, the catalyst provided in an exhaust passage is used for purifying the exhaust gas as well as increasing the exhaust gas temperature (the temperature at the high temperature side of the thermoelectric converting module) under the reaction heat. Under high load operation the exhaust gas flow from the engine can be split off and a part of it redirected through a by-pass which contains another catalyst (Related Art 2: JP-A-2000-352313). In order to protect the thermoelectric converting-element from exceeding its heat resisting temperature, e.g. during high load operation, the exhaust gas flow can also be redirected via a by-pass (Related Art 3: JP-06-081639).
Generally, in the exhaust system, when the catalytic temperature is low, for example, upon start-up of an engine, it has to be rapidly increased for smooth operation of the engine. Accordingly the exhaust emission catalyst is provided at a position in the exhaust system where the exhaust gas at high temperature (with high thermal energy) passes, for example, in the vicinity of an exhaust manifold and the like. Then an exhaust heat power generation apparatus is provided downstream of the exhaust emission catalyst in the exhaust system, for example, at a position near a sub-muffler. The temperature of the exhaust gas passing at the position downstream of the exhaust catalyst, however, becomes low because it has been used for increasing the catalytic temperature or it has been dispersed as it flows, resulting in decrease in the thermal energy. As a result, the thermoelectric conversion efficiency of the exhaust heat power generation apparatus is decreased, failing to effectively collect the thermal energy.
Under a high load of the engine (at a high engine speed), the exhaust emission catalytic temperature becomes considerably high as it is heated by the exhaust gas at high temperature in the vicinity of the exhaust manifold. When the catalytic temperature exceeds the activated temperature, its purification effect is deteriorated, which may thermally degrade the catalyst. Conventionally in the exhaust system, when the catalytic temperature exceeds the activated temperature, the engine is operated in a fuel-rich state so as to decrease the catalytic temperature. This may increase the fuel supply quantity, resulting in deteriorated fuel efficiency.